The present invention relates generally to polymer extrusion apparatus, and, more particularly, to multi-staged rotary screw extruders for polymer extrusion.
A variety of extrusion apparatus for injection molding thermoplastic materials are known in the prior art. Examples of such extrusion apparatus include U.S. Pat. No. 3,023,456 to Palfey, U.S. Pat. No. 4,155,655 to Chiselko et al., U.S. Pat. No. 4,185,060 to Ladney, Jr., and U.S. Pat. No. 5,597,525 to Koda et al. As shown in such patents, a common method for extracting vapors and gases during the extrusion process is to use a vented extruder. Venting is done in an extraction section or sections, and it can be to low pressure or to a vacuum. After the venting, a final pumping-section must develop pressure according to the load at the extruder exit. It must carry the flow rate of the extruder and build the pressure (from vacuum or low pressure) to the required exit pressure (e.g. die pressure required for extrusion).
In the final extraction section, where vapors are released to vacuum or low pressure, the polymer traveling in the channel must include an unbounded surface so that the gases may escape and travel to a vent port in the extrusion screw or in the barTel. This unbounded surface requires that the screw channel be only partially filled with polymer, and this unbounded surface will continue into the final pumping-section for a distance that is dependent upon the exit pressure of the extruder. At some distance along the length of the screw, the unbounded surface ends and the screw channel(s) are filled such that polymer traveling in the channel is fully bounded. The screw channels in the final pumping-section are completely full of polymer.
The axial length of the final pumping-section that is completely full of polymer is proportional to the exit pressure being developed. If the exit pressure is low, then the pumping section will have a very short length of fill. As the pressure is increased (greater load on the extruder), the length of the final pumping-section that is completely fall will become longer.
Theory and practice have shown that the optimum final pumping-section for a two-stage extruder (a final pumping-section that builds the pressure in the shortest length of the extruder) has a channel depth that is greater than that of the metering section (in the first stage of the extruder). For a square pitched screw as shown in U.S. Pat. No. 3,023,456 to Palfey (channel lead length equal to barrel diameter, channel lead length being the axial distance between flights), the pumping section depth must be 50% greater than the metering-section depth of the first stage for optimum pressure development. The so defined optimum channel-depth of the final pumping-section maximizes the pressure capability of the extruder. Subsequent vented extruder designs, such as those mentioned above, have specifically stated and followed the configuration of a single channel-depth final pumping-section as given by Palfey. Further, none of such prior art designs address the issue of pressure stability.
Pressure stability is addressed for extrusion in U.S. Pat. No. 4,867,927 to Funaki et al. and U.S. Pat. No. 3,712,594 to Schippers et al. In the apparatus taught by Funaki et al. (which does not include venting) a flow restriction is added to the screw to dampen the flow oscillations. A valve mechanism on the axis of the screw in the screw core and controllable from outside the extruder is used by Schippers et al. to dampen the flow oscillations. Screws with metering sections are used for plasticating with features that do not include venting or degassing. Specifically, reference U.S. Pat. No. 3,698,541 to Barr and U.S. Pat. No. 4,049,245 to Tadmor et al.. Both appear to show barrier type screws that include a constant channel-depth final pumping-section. However, neither Barr nor Tadmor et al. utilize the double channel-depth final pumping-section, or address stability of flow or pressure.
In a non-continuous process (as opposed to a continuous process for which venting here is required) taught in U.S. Pat. No. 4,648,827 to Laimer et al., a screw for injection molding does use a tapered channel-depth in the final pumping-section of the screw. However, Laimer et al. teaches a non-vented design, and the channel depth is not two distinct values. Additionally, since Laimer et al. is a non-continuous injection molding application, flow stability is not a concern.
The flow of polymer in a vented multi-stage extruder has inherent instability, typically in but not limited to, the solids conveying and melting processes therein. As the flow enters the final pumping-section, flow rate variation will cause the inventory of polymer in the final pumping-section to fluctuate. This fluctuation of inventory results in a change in the fill length of the final pumping-section. Since the final pumping-section fill-length is a factor in determining the exit pressure from the extruder, the exit pressure will also vary accordingly. Exit pressure instability in proportion to final fill-length variability of the final pumping-section is the end result. Thus, the prior art fails to teach a vented screw design in which the final pumping-section of the screw includes two or more channel depths in order to provide flow stability in continuous extrusion operations.
It is therefore an object of the present invention to provide a vented, multi-staged rotary screw extruder for polymer extrusion that allows for flow stability in continuous extrusion operations.
Another object of the present invention is a vented, multi-staged rotary screw extruder wherein the final pumping-section of the screw includes two or more channel depths in order to provide flow stability in continuous extrusion operations.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing a vented, multi-staged rotary screw extruder with a final pumping-section of the screw that includes two or more channel depths. Two or more channel depths in the final pumping-section of a multi-staged vented extruder improves exit-pressure and flow stability. The channel-depth of the first portion of the final pumping-section is deeper than the second portion of the final pumping-section. The axial length of the first portion of the final pumping-section is preferably about two-thirds (⅔) of the final pumping-section length. The deeper first channel-depth acts as a reservoir resulting in smaller fluctuations in axial final fill-length as the input flow to the final pumping-section varies. The second channel-depth in the remaining approximate one-third (⅓) of the total pumping-section is preferably sized so as to optimize pressure development. Therefore, exit-pressure stability is achieved by the depth of the first channel in the final pumping-section, and optimum pressure development is produced by the depth of the second channel in the final pumping-section thereby providing flow stability in continuous extrusion operations.